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In this standard, they define four levels of transaction isolation with different possible outcomes for the same transaction mixes.That is, the same work performed in the same fashion with the same inputs, may result in different answers given your isolation level. You are permitted to read uncommitted 'dirty' data.This last point is a frequently misunderstood concept and a small demonstration will clear it up. They just executed as if they were the only transaction in the database at that point in time.The following table represents two sessions performing work over time. No matter how many times Session 1 queried table B - the count would be the count that was committed in the database at time .
It does not imply that there is some serial ordering of the transactions that would result in the same outcome.
Serializable This is generally considered the most restrictive level of transaction isolation, but provides the highest degree of isolation.
A serializable transaction operates in an environment that makes it appear as if there are no other users modifying data in the database, the database will be "frozen" at the point in time your query began.
It may have disappeared, it may have been updated, and so on. This means that if you executed a query at time T1, and re-execute it at time T2, additional rows may have been added to the database, which affects your results.
This differs from the non-repeatable read in that in this case, data you already read has not been changed but rather that more data satisfies your query criteria than before.